As is well known in the art, poly(2,6-di-substituted-1,4-phenylene oxide)s are prepared by oxidative polymerization of 2,6-di-substituted phenols, and they exhibit high heat resistance. For instance, there are reported poly(2,6-dimethyl-1,4-phenylene oxide), in J. Am. Chem. Soc. 81, 6335-6336 (1959), and poly(2,6-diphenyl-1,4-phenylene oxide), in Macromolecules, 2, 107-108 (1969). The reason to use phenols having substituents at the 2 and 6 positions is that coupling at the two ortho positions is blocked, as is set out in J. Polym. Sci.: Part A: Polymer Chemistry, 36, 505-517 (1998).
On the other hand, with respect to the oxidative polymerization of 2,5-di-substituted phenols, which have no substituent at one of the ortho positions, there are reported a method using a copper/tetramethylethylenediamine catalyst, in Ecletica Quim., 18, 93-100 (1993); a method using a copper/dimethylpyridine catalyst, in Polymer, 20(8), 995-1002 (1979); a method using a copper/monoalkylamine catalyst, in Chem. Prum., 22(9), 451-454 (1972); a method using a copper/dialkylamine catalyst, in Polimery, 14(11), 535-538 (1969); a method using a manganese/alkoxide catalyst, in JP-B-47-619 (“JP-B” means examined Japanese Patent Publication); a method using a basic copper/pyridine catalyst, in JP-B-50-28999; and a method using a manganese/salicylaldehydeimine, in JP-B-48-20239.
However, the polymers obtained according to the methods using these catalysts are not crystallized after having once been melted. In general, it is known that, when a polymer is crystallized after melting, the heat resistance of the melt molding is kept to a level of its crystal melting point, and the solvent resistance is improved. Accordingly, since the polymers obtained by use of the known catalysts are not crystallized after melting, there arises the problem that the inherent heat resistance and solvent resistance cannot be fully developed. Presumably, this is because, with the prior art catalysts, side reactions, such as a coupling reaction at the ortho positions, an oxygenation reaction, and the like, cannot be suppressed to a satisfactory extent, so that the resultant polymer contains, in large amounts, various structures other than the 1,4-phenylene oxide structure.